The invention pertains to a substrate that has an outer layer of a superhard material, as well as a method of making the same. More specifically, the invention pertains to a cutting tool, as well as a method of making the same, that has an adherent outer layer containing boron and nitrogen which applicant believes to be boron nitride wherein the predominant crystalline form is cubic boron nitride (cBN).
As materials technology advances many new materials, including new hard materials, become commercially useful. Such new hard materials include without limitation sintered ultra-fine powdered metals, metal-matrix composites, heat treated hardened steels (hardnesses of between 50 to 65 Rockwell C), and high temperature alloys. These new materials have extraordinary combinations of properties, such as, for example, hardness, toughness and wear resistance, that make them very suitable for uses in heavy industries, aerospace, transportation, and consumer products.
For these new hard materials to realize their optimum commercial potential, one must overcome the challenges these materials present to existing manufacturing and finishing processes. One of the reasons these challenges exist is that these materials are very difficult and expensive to drill, cut and form. One can best address these challenges by the use of strong cutting tools that use a superhard coating.
At the present time, the two primary superhard commercial cutting tools comprise a polycrystalline diamond (PCD) cutting tool and a polycrystalline cubic boron nitride (PCBN) cutting tool. Diamond has a Knoop 100 hardness of between about 75 GPa and about 100 GPa. Cubic boron nitride has a Knoop 100 hardness of about 45 GPa.
The PCBN cutting tools typically find application in the machining of hard ferrous materials. The PCD cutting tools have their typical application in the machining of hard non-ferrous alloys and difficult-to-cut composites. In the typical polycrystalline (PCD or PCBN) cutting tool, the cutting edge comprises a superhard tip brazed onto a carbide blank. The tip comprises micrometer sized diamond or cubic boron nitride (cBN) crystals intergrown with a suitable binder and bonded onto a cemented carbide support. The crystals and blanks are synthesized and sintered under high pressure-high temperature (HP-HT) conditions such as, for example, 50 kbar and about 1500.degree. C.
The HP-HT manufacturing process, as well as the finishing process for these tips, each entails high costs. The result is that PCD cutting tools and PCBN cutting tools are very expensive. In addition to the expense, these cutting tools usually comprise a single tipped tool wherein the tip has relatively few styles with a planar geometry. Even though these cutting tools are expensive and come in relatively few styles, for the present time they are the best (and sometimes the only) cutting tool suitable to economically machine these new hard difficult-to-cut materials.
Through the development of techniques for the low pressure deposition of diamond one is able to deposit conforming layers (or films) of diamond on cutting tool substrates without any significant limitation to the geometry of the cutting tool. While the diamond-coated cutting tools have advantages over the PCD cutting tools, there remain some significant limitations to the use of diamond coated cutting tools.
One primary limitation with diamond cutting tools (i.e., PCD and diamond coated cutting tools) is that diamond oxidizes into carbon dioxide and carbon monoxide during high temperature uses. Another principal limitation with diamond cutting tools is the high chemical reactivity of diamond (i.e., carbon) with certain materials. More specifically, materials that contain any one or more of iron, cobalt, or nickel dissolve the carbon atoms in diamond. These limitations reveal that while diamond cutting tools provide certain advantages, there is a universe of materials that require a cutting tool with a superhard coating, but for which the use of a diamond cutting tool is inappropriate.
It is very apparent that there is a need to provide a cutting tool with an adherent superhard coating that overcomes the above problems extant with diamond-coated cutting tools. More specifically, there is a need to provide a cutting tool with an adherent superhard coating wherein the coating does not oxidize during high temperature use. There is also a need to provide a cutting tool with an adherent superhard coating wherein the coating does not chemically react with workpiece materials that contain any one or more of iron, cobalt, or nickel.
One superhard material that passivates by the formation of a protective oxide at high temperatures is boron nitride. In addition, boron nitride does not chemically react with any one or more of iron, nickel, or cobalt at typical metalworking temperatures. As a consequence, a workpiece which contains any one or more of iron, cobalt, or nickel does not dissolve the boron nitride. These advantageous properties of boron nitride exist with respect to all of the crystalline forms of boron nitride; however, those forms of boron nitride which exhibit a high hardness so as to provide for a superhard coating comprise the cubic boron nitride (cBN) and the wurtzitic boron nitride (wBN) crystalline forms of boron nitride wherein cBN has especially good properties and is the preferred crystalline form. It is to be expected, however, that a coating which is predominantly cBN will have some other crystalline forms of born nitride (e.g., amorphous boron nitride (aBN) and hexagonal boron nitride (hBN), as well as aBN contained therein and still exhibits excellent hardness, resistance to chemical reactivity, and passivation characteristics.
It thus becomes apparent that a boron nitride-coated cutting tool, especially a cBN-coated cutting tool, would possess highly desirable properties for the machining of new hard difficult-to-cut materials at high temperatures. It is also apparent that a boron nitride-coated cutting tool, especially a cBN-coated cutting tool, would possess highly desirable properties for the machining of new hard difficult-to-cut materials that contain one or more of iron, cobalt, or nickel.
The literature contains a number of articles that address the deposition of a thin film (or layer) of cBN onto a substrate in conjunction with the use of interlayers.
For example, the article by Murakawa et al. entitled "Characteristics of C-BN Film Made by a Reactive Ion Plating Method", pages 1009-1104 suggests two different basic approaches to the use of interlayers between the substrate and the cBN outer coating. One approach is to use a buffer interlayer alone that present a gradient of boron and nitrogen. The article suggests the post-treatment annealing of this coating scheme. The other approach is to use a titanium interlayer between the above buffer layer and the substrate.
The article by Gissler entitled "Preparation and Characterization of Cubic Boron Nitride and Metal Boron Nitride Films", Surface and Interface Analysis, Vol. 22, (1994), pp. 139-148 suggests the sequential deposition of multiple layers of Ti/BN with a subsequent annealing treatment. The Gissler article also suggests the addition of elements to the boron and nitrogen, such as, for example, titanium to form a Ti--B--N compound. However, the reference in Gissler to a coating within the system Ti--B--N--C having high hardness deals with a TiB.sub.2 coating that does not use interlayers. Mitteier et al., "Sputter Deposition of Ultrahard Coatings Within the System Ti--B--C--N", Surface and Coatings Technology, 41 (1990), pp. 351-363.
The article by Ikeda et al. entitled "Formation and characterization of cubic boron nitride films by an arc-like plasma-enhanced ion plating method", Surface and Coating Technology, 50 (1991), pp. 33-39 suggests a coating scheme with a titanium base layer, a layer of boron and nitrogen with a concentration gradient that moves from a boron rich-BN (iBN) composition next to the titanium layer to a constituent BN next to the outer layer, and an outer layer of cBN. Ikeda et al. suggests the use of ion plating techniques to deposit the iBN layer.
The patent literature also presents a number of patents that discuss the deposition of a cBN layer onto a substrate in conjunction with the use of interlayers. In this regard, U.S. Pat. No. 4,731,303 for a CUBIC BORON NITRIDE COATED MATERIAL AND PRODUCING METHOD OF THE SAME to Hirano et al. suggests the use of an interlayer of the nitrides or nitroxides of any one or more of Al, Ga, In, and Tl. U.S. Pat. No. 4,892,791 and U.S. Pat. No. 5,137,772 for a BODY COATED WITH CUBIC BORON NITRIDE & METHOD FOR MANUFACTURING THE SAME to Watanabe et al. each show the use of interlayers that include nitrides or borides of the Group IVb, IIIb, Vb, IVa, Va, and VIa elements.
The above documents present various approaches to the deposition of a superhard coating containing boron and nitrogen. However, there remains a need to provide an approach that deposits such a coating that has a sufficient thickness so as to be useful as a cutting tool and as a wear product.
It thus becomes apparent that in addition to providing a cutting tool with an adherent superhard coating, it would be desirable to provide a cutting tool with an adherent superhard coating, which preferably contains boron and nitrogen and more preferably, contains cBN, wherein the coating has a sufficient thickness so as to provide an adequate useful life.